I've noticed that it explicitly states in some spells, such as Dissonant whispers which has a verbal component only, that it can only be heard by the target
The caster would whisper a cacophonous sound that only the target could hear
However I have wondered if a Bard can subtly cast any spell in their repertoire against any person without a high enough knowledge in Arcana or personal knowledge of the caster.
Considering the following is true...
A bard can fulfil the V, S & M components of a spell by simply playing their instrument.
- M, instrument as spellcasting focus
- S, movement of playing the instrument
- V, sound of the instrument or accompanied song
So the conditions for spells here are an everyday action and the resulting effect is often physic, or mental not necessarily visible to others.
A good example would be Hideous Laughter (VSM). If my Bard were to play this and a target was to fall over laughing, I don't think it would be obvious a spell was involved.
This is very different to a Mage Kamehamehaing a fireball at someone.
To be perceptible, the casting of a spell must involve a verbal, somatic, or material component.
Most bards aren't magic.
True bards are not common in the world. Not every minstrel singing in a tavern or jester cavorting in a royal court is a bard.
It seems to be implied their magic is subtle by nature.
Bards say that the multiverse was spoken into existence, that the words of the gods gave it shape, and that echoes of these primordial Words of Creation still resound throughout the cosmos. The music of bards is an attempt to snatch and harness those echoes, subtly woven into their spells and powers.
This question has been asked for Vicious Mockery Can a bard cast Vicious Mockery without passersby thinking it's an attack? but not in a more general context, plus the answers there seem very subjective.